Final answer:
By setting the equations of the congruent vertical angles equal to each other (3x = 120), we can solve for x and find that x equals 40.
Step-by-step explanation:
If ∠2 and ∠4 are vertical angles, and the measure of ∠2 is given as 3x degrees and the measure of ∠4 is given as 120 degrees, then we can find the value of x by setting these measures equal to each other since vertical angles are congruent.
The equation is: 3x = 120.
To solve for x, you would divide both sides of the equation by 3.
x = 40
Therefore, the value of x is 40, which corresponds to answer choice (b).